Adaptive acoustic detector apparatus

ABSTRACT

An adaptive acoustic detector provides a signal for switching on the power supply of approximately positioned transmitter upon the detection of desired information which may comprise either a continuous or an impulse acoustic signal, irrespective of the attendant background noise. A composite signal composed of both the desired information and background noise is separated by a background insensitive amplifier which selectively feeds back only the background information thereby allowing the unattenuated passage of the desired information. An appropriate detector, upon the receipt of the desired information, permits the passage of a signal indicative thereof to provide the above mentioned switching signal. The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

United States Patent [72] inventors Henry Naubereit Browns Mills, NJ.;David L. Baddorf, Hatboro, Pa. [21] Appl. No. 679,255 [22] Filed Oct.30, 1967 [45] Patented Mar. 9, 1971 [73] Assignee the United States ofAmerica as represented by the Secretary of Navy [54] ADAPTIVE ACOUSTICDETECTOR APPARATUS 11 Claims, 3 Drawing Figs.

[52] U.S. Cl. 340/16, 340/258, 340/261 [51] Int. Cl G081: 13/00 [50]Field ofSearch 340/3, 5,6, 15, 16, 16 (R), 16 (M), 258, 258 (D), 261

[56] References Cited UNITED STATES PATENTS 3,056,928 10/1962 Marks340/16-R 3,111,657 11/1963 Bagno 340/258 3,147,467 9/1964 Laakmann340/261 3,202,968 8/ 1965 Eady, Jr. et al. 340/5X 3,218,556 1l/1965Chisholm 343/17.1X

3 ,238 ,502 3/1966 Babcock 340/1 6-R 3,240,930 3/1966 Porter et al.340/l6-R 3,320,576 5/1967 Dixon et a1. 340/5 3,341,810 9/1967 Wallen340/16 Primary Examiner-Richard A. Farley Attorneys-G. J. Rubens andHenry Hansen ABSTRACT: An adaptive acoustic detector provides a signalfor switching on the power supply of approximately positionedtransmitter upon the detection of desired information which may compriseeither a continuous or an impulse acoustic signal, irrespective of theattendant background noise. A composite signal composed of both thedesired information and background noise is separated by a backgroundinsensitive amplifier which selectively feeds back only the backgroundin- 10 $52 r 'j I 43 F33 T0 42 DELAY CO/Vf/NUOUS XMITTER RESPONSIVE i IMODULATOR I DETECTOR L NETWORK I 22 l 3 l 31 Low 55 BACKGROUND 7 FILTERINSENslT/VE 13 BIASING DIFFERENCE BIASHNG I AMPLIFIER NETWORK AMEuzrwonx 2, r J EMITOTEERR sou. w 1 is I so 51 14 I 7 7 DELAY l me Icoulz RESPONSIVE 1 AMP. NETWORK 17 1 l T Low PASS l I J AGC. FILTER 9IMPULSE OR I mpur Lever- DETECTOR CONTROLLER AMP. J l 277 l XMITTERADAPTIVE ACOUSTEC DETECTOR APPARATUS BACKGROUND OF THE INVENTION Theinvention relates to a detecting apparatus for receiving acousticsignals and more particularly to a network of amplifiers, filters, anddetectors which selectively extract desired information from a wide bandof frequencies having associated therewith random or other backgroundnoise. These selective extractions are in effect target detections whichmay comprise, for example, vehicle movement and/or button bombdetonation which may be caused by enemy troop movement. Vehicle movementproduces a continuous audible signal while bomb detonation produces anaudible impulse. Both of these phenomena are detected by the apparatus.The attendant noise may be attributed to animal life, rustling of leavesor other foliage, or other random phenomena occurring in a jungleenvironment.

The present invention is contemplated for use in a jungle environment asan electronic spy to listen for and activate means for transmittingvehicle and/or troop movements. The invention may be secreted behindenemy lines and, once so secreted, may be unrecoverable. Consequently,it is paramount that transmission should be kept at a minimum to bothconserve power and avoid detection by the enemy. It is also paramountthat information, when transmitted, may reasonably be assumed to containthe desired vehicle and troop movement components rather than randomnoise alone. This assumption was not reasonable and in fact not possiblein the prior art as acoustic-sensing apparatus were heretofore adjustedto indicate detection only when the ambient sound level exceeded apreselected threshold. Due to the wide variations in backgroundconditions, however, no practical fixed setting was plausible. Too higha threshold resulted in the loss of targets while too low a thresholdrendered the detector overly sensitive to random noise. As loss of atarget could not be tolerated, prior art devices were biased at lowthreshold levels thereby to insure the receipt of all targetinformation. As a result, many of the targets detected by these acousticsensors were false. Much undesired information was obtained and anunacceptable number of false alarms occurred.

Moreover, the power supplies associated with the respectivetransmitters, being almost always on, were rapidly depleted thusrendering the acoustic-sensing apparatus useless.

it is thus apparent that an acoustic detector which rejects unwantednoise information, enhances the receipt of desired signal information,and provides a signal in response thereto for switching on a transmitterwith a high degree of probability that desired information will betransmitted, is highly advantageous.

SUMMARY OF THE INVENTION Accordingly, it is the general purpose of thisinvention to provide an acoustic detector adapted for use in a jungleenvironment and capable of detecting vehicle movement and/or button bombdetonation with a minimum of false indications from environmentalconditions.

in brief, an acoustic sensor receives a composite signal composed ofboth desired information and background noise. This composite signal isfed into a background insensitive amplifier which selectively feeds backonly the background information, thereby allowing the unattenuatedpassage of the desired information. It is noted that this desiredinformation may comprise either a continuous signal, as from vehiclemovement, or an impulse signal, as from a button bomb detonation. If theformer occurs, an appropriate detector permits the passage of a signalindicative thereof after a predetermined time delay to actuate the powersupply of a transmitter. Similarly, if an impulse is received, it alsois detected and a signal indicative thereof is passed, the power supplyagain being activated after a predetermined time delay. Suitablefeedback is provided between the two detectbrs to insure that thecontinuous detector will operate only upon the receipt of a continuoussignal LII and the impulse detector will operate only upon the receiptthereto of an acoustic impulse. Further controls are provided withinboth the continuous detector and the impulse detector to insure that thesignals received are in fact indicative of either vehicle movement orimpulse phenomena and not merely noise transients or the like. The timedelay of the continuous detector is sufficiently long so as to insurethat the device will not trigger on transient phenomena which it mayreceive. Similarly, the time delay in the impulse detector affordsfurther protection from unwanted signals by assuring that an impulse ofsufficient duration is present before an output pulse is generated.

It is noted that the word impulse" as used herein connotes a signal ofrelatively short duration and is not meant to comprise the classicalelectrical definition, namely a signal of infinite magnitude occurringin zero time.

BRIEF DESCRIPTION OF THE DRAWING F IG. 1 is a diagram of the apparatusshowing the various components thereof in block diagram form;

FIG. 2 is an electrical representation of a portion of the continuousdetector shown in FIG. 1; and

FIG. 3 is a graph of the general output of the background insensitiveamplifier shown in F IG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing,and more particularly to FIG. 1, there is shown an adaptive acousticdetector apparatus 10 having at its input an acoustic sensor 11, whichmay be a microphone. The acoustic sensor 111 is connected to abackground insensitive amplifier shown generally at 12. This amplifierhas four outputs, one of which goes to a transmitter modulator. Twooutputs, l3 and 14, respectively, serve as input signals to a continuousdetector 15 and an impulse detector 16. The output shown at 27 alsoserves as in input signal to continuous detector 15. Each of the:aforementioned detectors provides, respectively, signals l7 and 18 whichserve as input signals to an OR gate or other suitable logic device 19.The output of the system 20 is taken at the output of this lastmcntionedlogic device 19.

As mentioned heretofore, a signal is received by acoustic sensor 11.This signal may comprise desired and undesired information over a widerange of frequency distributions. The sensor 11 is preselected, however,to have a flat frequency response from 40 Hz. to 2500 Hz. Thus, anysignal information occurring below or above this frequency band isseverely attenuated within the sensor ll itself. The output signal ofthe sensor 11 is, however, still of the broadband type in that it iscomprised of signal information over a wide frequency range, namely from40 Hz. to 2500 Hz. The broadband signal thus derived is amplified by anysuitable operational amplifier 2l. The output signal from amplifier 21is. fed both to a low-pass filter 22 and to a transmitter modulator (notshown). The modulator will not operate upon the receipt of this signalas the transmitter power switch, associated therewith, is initially inthe OFF position and will turn ON" only upon the receipt of a signalderived from the output 20 of the acoustic adaptive detector 10, in amanner to be hereinafter described.

The low-pass filter 22 passes all low frequencies up to and including200 Hz. with minimum attenuation. However, this filter is 35 db down at300 Hz. Thus, severe attenuation occurs above 200 Hz. The purpose offilter .22 is to reject the unwanted and troublesome signals occurringabove 200 Hz. and thereby to restrict the information passed toamplifier 23. Two hundred Hz. was chosen as the cutoff frequency forfilter 22 as prior testing and evaluation has shown that the desiredvehicle movement and button bomb impulse detonation frequencies occurbelow this level. Further, it is noted that the major false alarmsdictated by the prior art occurred above 200 Hz.

Amplifier 23 is of any desired type and amplifies those signals below200 Hz. only, the signals occurring above 200 Hz. having been filteredout by low'pass filter 22. The output from amplifier 23 is fed to bothan emitter follower 24 and a low-pass automatic gain control (AGC)filter 25. The AGC filter 25 output is fed to an AGC amplifier 26 andthrough line 27 is fed to one side of continuous detector 15. The AGCfilter 25 and amplifier 26 provide feedback to the inputs of bothamplifier 21 and amplifier 23. The purpose of this AGC feedback loop isto insure a constant amplitude signal output at point 28. This feedback,in effect, adjusts the gain of amplifier 23 with respect to thebackground or undesired information only, thereby preventing the desiredinformation from affecting the system gain.

This result may be more readily understood by referring to FIG. 3. Thisfigure is a graphical representation of the signal at the output ofamplifier 23. The amplitude thereof is plotted on the ordinate axis,while the frequency is plotted on the abscissa. Referring now to thisgraph it is seen that vehicle movement and/or button bomb impulsedetonation generally occur in the range between 80 Hz. and 200 Hz.Background noise, however, though present at all frequencies, is mostpredominate in the range from 40 Hz. to 80 Hz. Therefore,

. AGC filter 25 is designed to pass frequencies below 80 Hz. but

to reject those signals between range 80 Hz. to 200 Hz. Thus, thatportion of the signaloccurring at the output of amplifier 23 whichcontains undesired noise is amplified by AGC amplifier 26 and fed backto the inputs of, respectively, amplifiers 21 and 23 to reduce thebackground gain thereof without reducing the gain of the desiredinformation.

Referring again to FIG. 1, it is seen that the output signal at point 28from amplifier 23 is fed both to a continuous detector and an impulsedetector 16 through an emitter follower 24, or the like, havingrespective outputs l3 and 14. The operation of these detectors will nowbe described.

If a continuous signal, indicative of vehicle movement, appears at theoutput 13 of emitter follower 24, it will be amplified by suitableamplification means 29 within continuous detector 15 and passed tobiasing network 30. The frequency content of this signal may varyanywhere from 40 to 200 Hz. That is, the frequency content of thissignal is the same as that appearing at the output 28 of amplifier 23.If vehicle movement is not present, the frequency content of the signalappearing at the input of biasing network 30 will vary only from 40 to80 Hz. However, if vehicle movement is present, frequencies up to 200Hz. will be generated.

As noted heretofore, one side of continuous detector 15 is fed by anoutput signal from AGC filter 25 through line 27. This signal feedsbiasing network 31. It is noted, however, that the frequency content ofthis signal may vary only between 40 and 80 Hz. since the AGC filter 25will not pass signals above this range. If vehicle movement is notpresent, biasing networks 30 and 31 are balanced and the differenceamplifier 32 is quiescent. However, if a continuous signal from vehiclemovement is present, it will upset this normally balanced condition andthe resultant imbalance between the two input signals will causedifference amplifier 32 to provide an output to both delay responsivenetwork 33 through line 42 and, through line 34, to the input of aninput level controller 35, in impulse detector 16. Line 34 is in fact afeedback line between the two detectors. The purpose of this feedbackwill be later described.

Referring to F IG. 2, the operation of the continuous detector will bedescribed in more detail. Biasing networks 30 and 31 is fact comprise acombination of resistors, capacitors, and diodes suitably connected toprovide an effect known as voltage doubling. If a continuous signal fromvehicle movement is not present, then a balanced condition appears atthe two inputs 40 of difference amplifier 32. The difference amplifier32 is shown, in part, as two transistors 52, each receiving an inputsignal at their respective bases 40 from biasing networks 30 and 31. itis noted that the signal appearing at the output of amplifier 29 is inphase with the signal from the AGC filter 25 output signal. When thesesignals go positive" capacitors 36 charge up to a predetermined level.When the signals go negative capacitors 36 generate a current anddischarge through diodes 37 and charge capacitors 38. Diodes 53 preventcapacitors 36 from discharging to ground. The capacities of capacitors38 are greater than those of capacitors 36. Therefore, capacitors 38 arenot fully charged by the first discharges of capacitors 36. Rather, theycontinue to charge until further positive going signals are appliedthereto in the manner above described. At this point, when the signalsagain go negative, capacitors 38 discharge through respective resistors39, thereby applying equal bias to each of the bases 40 of the twotransistors 52. As long as this condition persists, ON/OFF switch 41,which may itself be a transistor, a plurality thereof, or other activeelement or elements, remains in the OFF" condition and no signal appearsat the output of difference amplifier 32.

If, on the other hand, a signal appears which is between Hz. and 200Hz., then the balance created within the biasing networks 30 and 31 isno longer present. For, while the noise from 40 Hz. to 80 Hz. is alsopresent with the greater than 80 Hz. signal, and is balanced out by therespective biasing networks 30 and 31, there is nothing to balance outthe greater than 80 Hz. signal. Therefore, the transistors will nolonger be in balance and ON/OFF switch 41 will be turned ON" therebycausing a signal to appear at the output of difference amplifier 32.

This output signal, fed by line 42 to delay responsive network 33 and byline 34 to the input of input level controller 35, will in general beindicative of vehicle movement in the field. However, and thoughunlikely, it is possible that a random transient or other noise may haveproduced such an imbalance in difference amplifier 32. To protectagainst this possibility, the delay responsive network 33 is provided.

Prior testing has shown that a random transient, though it may occurbetween 80 Hz. and 200 Hz., will exist for a much shorter duration thanthe signal produced by a continuously moving vehicle. Thus, the delayresponsive network 33 is provided to insure that a signal appearing atthe output of dif-. ference amplifier 32 must last for at least 4seconds before a signal is passed by the delay responsive network 33 tothe OR gate 19.

The signal from difi'erence amplifier 32 enters the delay responsivenetwork 33 via line 42 at point 43. The signal then passes throughresistor 44 and begins to charge capacitor 45. This RC network is theheart of the time delay portion of delay responsive network 33. Element46 is a four-layer siliconecontrolled switch which must receive acertain prescribed minimum voltage or trigger level before it willconduct. Capacitor 45 continues to charge through resistor 44 as long asthe signal is applied at point 43. If the signal is due to transient orother phenomena, it will decay long before the capacitor 45 has chargedto the point where controlled device 46 breaks down and conducts.However, if a continuous signal from vehicle movement is present,capacitor 45 will continue to charge and, at the trigger level ofelement 46, will discharge therethrough through resistor 47.

Diode 48 and capacitor 49 serve as a pulse stretcher to operate on theresultant pulse discharge from capacitor 45 such that it will be ofsufficient duration to enable OR gate 19. It is noted that resistor 47is chosen to have an ohmic value very much less than that of resistance44 such that the capacitor 45 will discharge through the resistance 47at a much more rapid rate than it charged through resistor 44. This willproduce a heavy burst of current sufficiently capable of enabling ORgate 19 and thereby turning on the transmitter power supply switch.

Referring again to FIG. 1, the impulse detector will now be described indetail. If an impulse signal appears at point 28 it will be transmittedby the emitter follower 24 and line 14 to the input of controlledamplifier 50 where it will be amplified and fed to both delay responsivenetwork 51 and input level controller 35. The input level controller 35is, in effect, an amplifier connected in the negative feedback mode withthe controlled amplifier 50. Controller 35 operates to reduce the outputlevel of controlled amplifier 50 at the rate of 50 db per second or 2.5db in 50 milliseconds. Moreover, the delay sponsive network 511 willemit a signal indicative of button bomb detonation only if a signalhaving a magnitude of at least 3 db persists for at least 50milliseconds. it can be seen therefore, that an impulse of at least 5.5db, which does not decay below 3 db in 50 milliseconds, must be presentat the input of controlled amplifier 5% before delay responsive network51 emits a pulse. Tests have shown that background impulse noiseoccurring in the range from 40 Hz. to 200 l-iz., while amplified bycontrolled amplifier 5t), is reduced substantially by controlleramplifier 35 such that the amplitude and duration thereof isinsufficient to cause delay responsive network 51 to emit a pulse.

Delay responsive network 51 is identical in schematic structure to thatof delay responsive network 33, the values of the components, however,being difierent. Network 51 is designed to provide a 50 milliseconddelay before it emits a signal. As noted above, prior testing has shownthat impulses due to background noise are far shorter than this timeinterval and are therefore not transmitted.

When a button bomb goes off, however, the amplitude thereof far exceedsthat of the background level and hence controlled amplifier 50 is driveninto a high gain condition, a condition which input level controller 35is not capable of reducing. if this signal lasts for at least 50milliseconds without dropping below 3 db, and a button bomb detonationwill so last, delay responsive network 51 emits a signal representativeof the button bomb impulse through line 18 to OR gate 19 and hence tothe transmitter power supply switch, thereby turning the transmitter on.

it is noted that the transmitted information from the transmittermodulator is coded such that upon receipt thereof it can be determinedwhether the desired information is produced by continuous or impulsephenomena. it was therefore necessary to provide feedback line 3dbetween difference amplifier 32 and input level controller 35 whichserves to provide a means for inhibiting the controlled amplifier 50 ofimpulse detector l6 upon the receipt of a continuous signal bycontinuous detector l5. Thus line 34 provides a feedback path forinhibiting the controlled amplifier 50 thereby, in effect, electricallydisengaging the impulse detector 16 from the apparatus and preventing itfrom operating upon the continuous signal received by continuousdetector 15. It is noted that the circuitry of continuous detector isnot susceptible to impulse phenomena.

it is contemplated that many of the above-described elements may befabricated as integrated circuits. For example, amplifier 2i mightreadily comprise a 10 terminal integrated circuit. Qne suchrepresentative type is SN-12l9 manufactured by Texas instruments. Also,while the above-described frequency cutoff points and periods of timedelay have been enumerated with some degree of particularity, it is tobe understood that these values may be varied as desired by one skilledin the art to provide optimum results. Moreover, it is contemplated thatthe invention may be used to listen for, and activate means fortransmitting, sounds other than those produced by continuous vehicles ortroop movements. For example, the invention could readily be utilized tolisten for enemy aircraft or missiles or the like that approach aposition below friendly radar.

Accordingly, the above-described arrangements are illustrative of theapplication of the principles of the invention only of a preferredembodiment for the practicing thereof. it will of course be recognizedthat numerous modifications and alterations may be made in theabove-described apparatus without departing from the spirit or scope ofthe invention as set forth in the appended claims.

We claim:

ll. An apparatus for detecting vehicle movement or troop movement in ajungle environment, comprising:

acoustic sensing means for providing an electrical signal within apredetermined range upon the receipt thereto of sound signals;

a background insensitive amplifier connected with and responsive to saidacoustic sensor means for selectively providing a first signal between afirst set of frequency limits and a second signal between a second setof frequency limits;

first detector means connected to said amplifier and receiving saidfirst signal and said second signal for providing a third signalindicative of said vehicle movement;

second detector means connected to said amplifier and receiving onlysaid first signal for providing a fourth signal indicative of said troopmovement; and

a logic gate connected to said first and said second detector means forproviding an output signal upon receiving either said third signal orsaid fourth signal.

2. The invention as defined in claim I wherein:

said logic gate output signal activates the power supply switch of aproximately positioned transmitter thereby to effect transmission of theelectrical signal provided by said acoustic sensing means.

3. The invention as defined in claim 1 wherein said backgroundinsensitive amplifier comprises:

first filter means operatively connected to said sensing means forpassing a first preselected portion of said electrical signal therebyproviding said first signal; and

second filter means operatively connected to receive said first signalfor feeding back a predetermined portion thereof to said first filtermeans and for passing a second predetermined portion, said secondportion being said second signal.

4. The invention as defined in claim 3 wherein said first detector meanscomprises:

first means responsive to said first signal passed by said first filtermeans;

second means responsive to said second signal passed by said secondfilter means;

difference amplifier means interposed between and responsive to saidfirst means and said second means for providing a signal when said firstmeans receives a signal different from that received by said secondmeans; and

delay means responsive to the signal produced by said differenceamplifier means for providing said third signal to said logic gate aftera predetermined time delay.

5. The invention as defined in claim 3 wherein said second detectorcomprises:

first amplifier means responsive to said first signal passed by saidfirst filter means;

second amplifier means connected to said first amplifier means forreducing the output level thereof at a predetermined rate; and

delay responsive means connected to said first amplifier means and saidsecond amplifier means for providing said fourth signal to said logicgate upon the expiration of a predetermined time delay.

6. The invention as defined in claim 3 wherein said backgroundinsensitive amplifier further comprises:

first amplifying means connected between said sensing means and saidfirst filter means for amplifying said electrical signal;

second amplifying means connected to the output of said first filtermeans for amplifying said first signal passed thereby;

automatic gain control means connected to the inputs of said firstamplifying means and said second amplifying means for maintaining thegain of said second amplifying means at a constant level; and

an emitter follower connected to said second amplifier means forproviding signals to said first and said second detector means.

7. The invention as defined in claim wherein said delay means comprises:

a first resistor;

a capacitor connected at one end thereof to said first resistor;

a second resistor; and

a silicone-controlled switch interposed between said connection and saidsecond resistor whereby said capacitor stores electrical energy for apredetermined time as governed by said first resistor and dischargessaid energy through said second resistor after having charged to apredetermined trigger level of said silicone-controlled switch.

8. Acoustic detecting apparatus, comprising:

acoustic sensing means receiving an acoustic signal;

first filter means operatively connected to said sensing 10 means forpassing a preselected portion of said signal;

second filter means operatively connected to receive the signal passedby said first filter means for feeding back thereto a predetenninedportion thereof and for passing a second predetermined portion;

' first detector 'means connected to receive only said signal passed bysaid first filter means for providing a first output signal upon thedetection of a first desired signal;

second detector means connected to receive said signal passed by saidfirst and said second filter means for providing a second output signalupon the detection of a second desired signal; and

a logic gate connected to said first and said second detector means forproviding a third output signal upon receiving either said first or saidsecond output signal. 9. The invention as defined in claim 8 whereinsaid second detector means comprises:

first means responsive to said signal passed by said first filter means;second means responsive to said signal passed by said second filtermeans; difference amplifier means interposed between and responsive tofirst means and said second means for providing a signal when said firstmeans receives a signal different from that received by said secondmeans; and

delay means responsive to the signal produced by said differenceamplifier means for providing said second output signal to said logicgate after a predetermined time delay.

10. The invention as defined in claim 8 wherein said first detectormeans comprises:

first amplifier means responsive to the signal passed by said firstfilter means;

second amplifier means connected to said first amplifier means forreducing the output level thereof at a predetermined rate; and

delay responsive means connected to said first amplifier means and saidsecond amplifier means for providing said first output signal to saidlogic gate upon the expiration of a predetermined time delay.

11. The invention as defined in claim 9 wherein said delay responsivemeans comprises:

a first resistor;

a capacitor connected at one end thereof to said first resistor;

a second resistor; and

a silicone-controlled switch interposed between said connection and saidsecond resistor whereby said capacitor stores electrical energy for apredetermined time as governed by said first resistor and dischargessaid energy through said second resistor after having charged to apredetermined trigger level of said silicone-controlled switch.

1. An apparatus for detecting vehicle movement or troop movement in a jungle environment, comprising: acoustic sensing means for providing an electrical signal within a predetermined range upon the receipt thereto of sound signals; a background insensitive amplifier connected with and responsive to said acoustic sensor means for selectively providing a first signal between a first set of frequency limits and a second signal between a second set of frequency limits; first detector means connected to said amplifier and receiving said first signal and said second signal for providing a third signal indicative of said vehicle movement; second detector means connected to said amplifier and receiving only said first signal for providing a fourth signal indicative of said troop movement; and a logic gate connected to said first and said second detector means for providing an output signal upon receiving either said third signal or said fourth signal.
 2. The invention as defined in claim 1 wherein: said logic gate output signal activates the power supply switch of a proximately positioned transmitter thereby to effect transmission of the electrical signal provided by said acoustic sensing means.
 3. The invention as defined in claim 1 wherein said background insensitive amplifier comprises: first filter means operatively connected to said sensing means for passing a first preselected portion of said electrical signal thereby providing said first signal; and second filter means operatively connected to receive said first signal for feeding back a predetermined portion thereof to said first filter means and for passing a second predetermined portion, said second portion being said second signal.
 4. The invention as defined in claim 3 wherein said first detector means comprises: first means responsive to said first signal passed by said first filter means; second means responsive to said second signal passed by said second filter means; difference amplifier means interposed between and responsive to said first means and said second means for providing a signal when said first means receives a signal different from that received by said second means; and delay means responsive to the signal produced by said difference amplifier means for providing said third signal to said logic gate after a predetermined time delay.
 5. The invention as defined in claim 3 wherein said second detector comprises: first amplifier means responsive to said first signal passed by said first filter means; second amplifier means connected to said first amplifier means for reducing the output level thereof at a predetermined rate; and delay responsive means connected to said first amplifier means and said second amplifier means for providing said fourth signal to said logic gate upon the expiration of a predetermined time delay.
 6. The invention as defined in claim 3 wherein said background insensitive amplifier further comprises: first amplifying means connected between said sensing means and said first filter means for amplifying said electrical signal; second amplifying means connected to the output of said first filter means for amplifying said first signal passed thereby; automatic gain control means connected to the inputs of said first amplifying means and said second amplifying means for maintaining the gain of said second amplifying means at a constant level; and an emitter follower connected to said second amplifier means for providing signals to said first and said second detector means.
 7. The invention as defined in claim 4 wherein said delay means comprises: a first resistor; a capacitor connected at one end thereof to said first resistor; a second resistor; and a silicone-controlled switch interposed between said connection and said second resistor whereby said capacitor stores electrical energy for a predetermined time as governed by said first resistor and discharges said energy through said second resistor after having charged to a predetermined trigger level of said silicone-controlled switch.
 8. Acoustic detecting apparatus, comprising: acoustic sensing means receiving an acoustic signal; first filter means operatively connected to said sensing means for passing a preselected portion of said signal; second filter means operatively connected to receive the signal passed by said first filter means for feeding back thereto a predetermined portion thereof and for passing a second predetermined portion; first detector means connected to receive only said signal passed by said first filter means for providing a first output signal upon the detection of a first desired signal; second detector means connected to receive said signal passed by said first and said second filter means for providing a second output signal upon the detection of a second desired signal; and a logic gate connected to said first and said second detector means for providing a third output signal upon receiving either said first or said second output signal.
 9. The invention as defined in claim 8 wherein said second detector means comprises: first means responsive to said signaL passed by said first filter means; second means responsive to said signal passed by said second filter means; difference amplifier means interposed between and responsive to first means and said second means for providing a signal when said first means receives a signal different from that received by said second means; and delay means responsive to the signal produced by said difference amplifier means for providing said second output signal to said logic gate after a predetermined time delay.
 10. The invention as defined in claim 8 wherein said first detector means comprises: first amplifier means responsive to the signal passed by said first filter means; second amplifier means connected to said first amplifier means for reducing the output level thereof at a predetermined rate; and delay responsive means connected to said first amplifier means and said second amplifier means for providing said first output signal to said logic gate upon the expiration of a predetermined time delay.
 11. The invention as defined in claim 9 wherein said delay responsive means comprises: a first resistor; a capacitor connected at one end thereof to said first resistor; a second resistor; and a silicone-controlled switch interposed between said connection and said second resistor whereby said capacitor stores electrical energy for a predetermined time as governed by said first resistor and discharges said energy through said second resistor after having charged to a predetermined trigger level of said silicone-controlled switch. 